Volume protection intrusion detection systems, such as burglar alarms, to detect intrusion in a substantially enclosed space, such as a room, are well known in the art. Typically the intrusion detection system includes presence and/or motion detectors. Two general types of detectors are used: passive and active. An example of a passive detector is a passive infrared detector which detects the presence and/or motion of infrared radiation generated by an intruder within a defined area to be protected. In the prior art, infrared intrusion detectors employ a plurality of segmented mirrors or lenses to gather infrared radiation from a plurality of spaced apart fields of view ("finger-like" projections) from a volume of space. A passive infrared intrusion detector employing a dome shaped fresnel lens and fresnel prism to refract and to focus radiation from a plurality of spaced apart fields of view from a 360 degree zone is disclosed in U.S. Pat. No. 5,017,783, assigned to the present assignee.
An example of an active detector is a microwave transceiver. The transceiver transmits and receives microwave radiation having frequencies greater than 1 Gigahertz, to detect the presence and/or motion of an object within the defined area to be protected. Microwave transceivers employing bent monopole antennas are also well known in the art. Microwave transceivers with bent monopole antennas are able to detect intruders in a volume of space, which is substantially conically shaped, with the axis of cone perpendicular to the plane of radiation of the monopole antenna. In addition, the prior art discloses a ceiling mounted microwave transceiver with 360 degree radiation pattern. See U.S. Pat. No. 5,023,594, assigned to the present assignee. The microwave radiation pattern as disclosed in that reference however, has a spatulate radial cross section pattern.
In the prior art, it is also known to use a single bipolar transistor as the active element in the oscillator portion of the microwave transceiver for transmission and detection of the reflected microwave radiation. However, in the prior art the single bipolar transistor is used with a dielectric resonator, which is expensive.
Finally, in U.S. patent application Ser. No. 07/817,339 filed on Jun. 1, 1992, and assigned to the present assignee, a single bipolar transistor with a UHF trimmer capacitor is used in the oscillator section of the microwave transceiver. However, in that application, a Schottky diode is used in the receiver section to mix the microwave radiation. A Schottky diode is an active element which can also be expensive.
An intrusion detection system employing dual technology, such as the combination of passive infrared and microwave are also known in the art. By using the combination of two detectors to detect the present of an intruder before an alarm signal is generated, false alarms are minimized. However, in order for the intrusion detection system using dual technology to operate properly, both detectors must be aligned to be directed at substantially the same volume of space. Since the volume of space to which each of the detectors is designed to protect may vary, the location of the dual technology intrusion detection system to protect the enclosed volume of space becomes important.
Thus, there is a need for an intrusion detection system, employing dual technology sensors, which is low cost, and which can be positioned virtually anywhere in an enclosed volume of space.